It is known that the spring assembly comprises at least two elastic units arranged in series and having different mechanical characteristics, such as torsional stiffness. The two elastic units are connected by a structure that has significant mass, which is the cause of undesired resonance.
In particular, an elastic unit can comprise a plurality of circumferential coil springs that torsionally connect the primary mass of the flywheel to the structure, the second elastic unit torsionally connecting the structure to the secondary mass and the structure being rotatable with respect to the primary and secondary masses.